Find the data sheet for the switchmode controller and design you powersupply based on the one on Elliots site... http://sound.westhost.com/project89.htm substituting in the **494 controller IC.... you will need to buy a pair of mosfets... the transistors used in a computer powersupply are not suitable... you will probably have to rewind the transformer, unless you can come up with a way to use it as it is, but in reverse.... hey, thats a good idea, I'm going to try that some time..

I think you should be alright.... the pins are different, so you'll have to work that out........ have a look at the 494 data sheet... It has a example circuit thing that you could use..... I think... why not just go any buy a SG3525?

you will need to buy a pair of mosfets... the transistors used in a computer powersupply are not suitable...

you'll probably find that those transistors *are* quite suitable, provided you don't expect to run get 250W+ from the PSU

The ones I've taken out of old PC PSU's are all high voltage BJT's rather than FET's - about 450v rated or so. I've used them in the past for high current drivers for coils , and they seem to work fine at 12v levels ( and they cope with the inductive spikes well too...)

you will almost certainly have to rewind the transformer - leave it overnight in acetone and the core will come apart without a fight. don't be tempted to get rough with it 'cos they're very brittle.

I'm currently using one as a high voltage driver for a bike CDI project, it works wonderfully in a 250v 50kHz inverter.

you should have no problem finding any number of xx494 designs to work with.

remember to use fast rectifiers on the output , normal ones will fry at the frequency you'll be running the thing at ( 20kHz+ , or the transformer isn't efficient).

have fun , don't be discouraged by all the components you will probably fry in the design process

check your waveforms on the scope - and then check 'em again.
you can't afford to get things wrong with a SMPS , when they go pop they do it very quicky.